The present invention generally relates to magnetic bearing designs incorporating high-temperature superconductors suitable for a variety of applications such as flywheel energy storage. More particularly, the present invention relates to magnetic beating designs including a permanent magnet assembly rotating above an array of high temperature superconductor files. Methods and apparatus are disclosed that optimize the arrangement of the high temperature superconductors so that the magnetic field that the rotating magnet experiences is more homogeneous. The increased field homogeneity reduces the eddy currents induced in the permanent magnet and improves the efficiency of the bearing. The high temperature superconductors are vertically stacked in a brickwall configuration in the preferred embodiments.
Storage of electrical energy is useful in a number of applications. Diurnal storage of electricity is important to electric utilities in order to efficiently utilize base load generating plants and to meet the varying load demands of their customers. For example, base-load plants can charge the storage units at night when demand is low, and then peak demands can be met by discharging the storage during the peak hours.
Energy storage can also play a substantial role in eliminating or postponing the installation of power lines with larger capacity. Power can be transmitted at night to a substation or user energy storage unit when demand is low, and then during peak power times, the energy storage units can be discharged. The placement of these energy storage units can occur in various parts of the electrical distribution system: utility parks where large amounts of energy can be stored; in tandem with photovoltaic or wind energy generation facilities that are time dependent; substation units; individual companies and houses. Energy storage units can be used on electric vehicles such as cars and buses, or as wayside energy storage for electric trains.
Flywheels are often considered for energy storage application. Their primary advantages are modularity, high energy storage density (Wh/kg), and high efficiency input and output of electrical energy. The ability to produce high strength flywheel rotors and the ability to efficiently transfer energy in and out of a flywheel are well known and will not be discussed herein.
The primary disadvantage of conventional flywheels is inefficiency in standby mode. These losses occur because the bearings that support the flywheel structure have high losses. High temperature superconductor bearings constructed in accordance with the invention have very low rotational loss and enable standby losses in flywheels to be 0.1%/hr or less. The high temperature superconductor bearings disclosed herein provide passive stability in all directions, i.e., they provide a positive stiffness in all displacement directions. Further, these novel designs allow rotational motion with very low friction.
A significant loss mechanism associated with a high temperature superconducting bearing is eddy currents. These eddy currents arise any time a normal conductor experiences a change in magnetic field. For high temperature superconducting bearings, the loss occurs as a component of drag torque that increases linearly with velocity. It was discovered by experiments in the inventors laboratory that some eddy currents occur in the rotating permanent magnet as a result of the discreteness of the high temperature superconductor elements of the array.
The high temperature superconductor elements contain gains that become magnetized and produce a levitating force on the permanent magnet. Unlike a permanent magnet, in which the magnetization is essentially uniform across the grain, the magnetization peaks toward the center of the high temperature superconductor gain (hereinafter called the "magnetic field maximum location"). To achieve large levitation pressure, large high temperature superconductor grains are desired. However, the larger the high temperature superconductor grain, the larger the peaking of the magnetization in the high temperature superconductor, and the more inhomogeneous the magnetic field that the permanent magnet experiences when it rotates over the high temperature superconductor.
This magnetic field inhomogeneity produces conflicting demands on the design of the high temperature superconductor gains. The gains should be large for large levitation pressure, but the grains should be small to produce a more homogeneous field on the permanent magnet. If one gigantic high temperature superconductor grain could be produced (a large single crystal), the homogeneity problem in the high temperature superconductor could be solved. The ability to produce such a single crystal is not currently available.
Eddy currents can also be produced due to inhomogeneities in the magnetic field of the permanent magnet. It is desirable to eliminate these eddy currents as well, although significant improvements in the homogeneity of the magnetic field produced by the high temperature superconductors yield significantly lower rotational losses.
Eddy current losses are also produced when an alternating current magnetic field is superimposed onto a direct current magnetic field. The effect of the eddy current loss on rotational drag is proportional to the product B.sub.avg B.sub.ac, where B.sub.avg is the average dc field and B.sub.ac is the amplitude of the magnetic field ac inhomogeneity superimposed on the dc field. Prior art designs have not succeeded in reducing these rotational losses to acceptable levels.
It is therefore an object of the invention to provide an improved low-loss bearing and method of use using a tiled array of high temperature superconductors.
It is another object of the invention to provide a novel high-efficiency flywheel energy storage device and method of use.
It is a further object of the invention to provide an improved rotational device and method of use utilizing large high temperature superconductor grains to achieve large levitation pressure, but to arrange these grains and high temperature superconductor array elements in such a way that the production of eddy currents in a rotating permanent magnet is minimized.
It is a still further object of the invention to provide a novel superconducting magnetic bearing and method of use using a plurality of superconductor pellets arranged in a staggered fashion to minimize magnetic field inhomogeneity.
It is yet another object of the invention to provide an improved superconducting magnetic bearing and method of use including superconducting material structures which are subdivided and arranged in a staggered arrangement.
It is a further object of the invention to provide a novel staggered, brickwall arrangement and method of use of superconducting material structures which reduce magnetic field inhomogeneity.
Other advantages and features of the invention, together with the organization and the manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the drawings.